Porting Scratch Everywhere! can be either extremely easy or extremely hard, depending on what your target platform is.
To start, simply create a new Makefile in the make/ directory, and add the target platform to the root Makefile.
If your target platform supports SDL2, you likely shouldn't need to change much. Firstly you'll likely need to configure some stuff in source/scratch/os.cpp, and if you're lucky, that should be it, if you're not well it should tell you the error!
If your platform doesn't support SDL2 you have a lot more work, firstly create a new folder in the source/ folder for your platform. Then create equivalent functions to the ones in source/3ds and source/sdl. You'll also need to add your platform in source/scratch/os.cpp just like with SDL2.
We do require a Dockerfile be present for all ports to make building easier. If you don't know how to make one, we can make one for you, just ask.
Please have a checklist, it makes it much easier to track progress on ports.